Selective replacement of metal sulfides



United States Patent SELECTIVE REPLACEMENT 0F METAL SULFIDES m... I.McGauley, Glen Cove, N. Y., Felix A.

Schanfelberger, Elizabeth, N. 1., and Edward S. Roberts, New York, N.Y., assignors to Chemical Construction Corporation, New York, N. Y., acorporation of Delaware No Drawing. Original application November 15,1949, Serial No. 127,452, now Patent No. 2,662,009, dated December 8,1953. Divided and this application September 11, 1953, Serial No.379,746

6 Claims. (Cl. 23-134) Roughly, the overall problem may be broken downinto two parts. First, the mineral values must be extracted from the oreby some successful leaching system. Second, pure metals must beeconomically and efiiciently recovered from the solutions obtained inthe leaching op erations. Whether considered as part of the leachingproblem, or as part of the metal-recovery problem, there is theadditional difficulty, which must be overcome, in that the mixture ofmetals or their minerals or salts must be separated each from the other.

In the past, largely with respect to the treatment of copper minerals, anumber of proposals for such chemical extraction procedures have beenpresented. Unfortunately, all these operations suffered from one or moreserious defects. As a result, none have been capable of commercialdevelopment on a scale commensurate with the possibilities of a suitableprocess of this type.

When the wide variety of problems presented by the copper industry aloneare considered, this failure is not particularly surprising. Forexample, it is desirable to be able to treat many varieties of copperminerals by the same process, including those of oxidized and sulfidecopper minerals. When this problem is complicated by the presence ofother minerals, which not only must be removed therefrom, but alsorecovered, the complexities of the problem and the previous failures toprovide successful solutions thereto become even more readilyappreciated.

Nevertheless, a demand remains in the industry for a process capable ofeasily and economically treating a minerals mixture. It must beadaptable to separate and to recover the copper and other metal valuesfrom the gangue and/or diluents. It is, therefore, a principal object ofthe present invention to provide such a process. It is with thecombination of leaching and minerals-separation that the presentinvention is concerned. In general, it deals with the treatment of oreswhich are mixtures of metal-values minerals to obtain one or moreenriched concentrates of metal sulfides. These may be either mixturesfreed from gangue and diluent metals, or individual selected sulfides.

Still more specifically, leach solutions, preferably obtained by acidleaching of the minerals mixture, containing dissolved salts of themetal values, are treated by a re- "ice placement. reaction. In thelatter are employed an additional metal sulfide or metal sulfides and areducing agent. The metal-values sulfides are thus precipitated in amixture of all the sulfides whose solubility is less than that of theadded metal sulfide. The mother liquor will contain, as dissolved salts,all the remaining metals-values, i. e., salts of those metals ,thesulfides of which are of equal or greater solubility than that of theadded sulfide. In addition, it will contain an amount of dissolved acidsalt equivalent to the added metal sulfide reacted in replacing thedesired sulfides. The desired sulfides, as precipitated by the reaction,are collected and treated to reconvert the sulfides to solutions ofsoluble salts. The latter may then be treated further to separateselected sulfides.

Of the previously-proposed processes, perhaps the best is that in whichcopper is separately removed as equivalents of crystallized coppersulfate from liquors obtained by acid leaching of minerals mixturescontaining iron and other metal minerals. In this previously proposedprocess an effective industrial method of separating soluble coppersalts from iron and other metal salts, in an acid leach solutioncontaining the copper and other metal sulfates, is disclosed. Theprocess involves treatment of the copper-bearing leach solution byreplacement with iron sulfides, usually that in the ore. When thisoperation is properly conducted, it results in the completeprecipitation of copper from the solution as copper sulfides. It isaccompanied by the simultaneous dissolution of an equivalent amount ofiron sulfides which is converted to ferrous sulfate. The copper sulfidesare collected by filtration and the filtrate is discarded. The collectedcopper sulfides are oxidized to copper sulfate in solution; and so muchof the latter as can be crystallized as purecopper sulfate is separatedout and collected. By recirculation, all copper is eventually thusrecovered.

This device was found to be very useful as a method of discarding ironand acid-insoluble diluent metals from a leaching-reduction systemwithout causing loss of copper. However, when the ore being treatedcontains other valuable metals, such as cobalt, nickel, zinc, manganeseand moylbdenum, these metals are also dissolved during leaching but arenot recovered. They remain in the leach solution and are removed fromthe system in the ferrous sulfate liquor. Since these metals arevaluable and are often present in amounts sufficient to be profitablyrecovered, they cannot be discarded with the ferrous sulfate withouteconomic loss. The problem of devising a suitable recovery system, whichwould also recover these additional non-ferrous metals, thus remainedunsolved. Preferably, each metal should be recovered as a separateproduct, if possible.

In accordance with the patent application, now United States LettersPatent No. 2,662,009, a process is provided for the separation andrecovery of these additional nonferrous metals, as their sulfides,alternatively as a mixture with the copper sulfides fraction or asseparated metal sulfides. This divisional application deals with thelatter. In the parent case it was shown that the mixed sulfides can beproduced from an ore concentrate containing non-ferrous metal valuestogether with acid-insoluble gangue and at least one diluent metal valuesuch as iron. The procedure comprises subjecting the ore concentrate toa leaching operation under oxidizing conditions in the presence ofaqueous acid, usually sulfuric acid, whereby substantially all the metalvalues, including the diluent, are dissolved as soluble salts in aqueousacid solution, removing any residual undissolved acid-insoluble solids,and subjecting the resultant clarified solution in a primary replacementunder acid conditions by simultaneous treatment with (a) a treatingsulfide of a metal such as iron sulfide the solubility of which sulfideat the treatment acidity is greater than that of the sulfide of anyother metal value to be recovered and (b) a reducing agent, continuingthe treatment until reaction substantially ceases, and collecting theresultant concentrated precipitate of sulfides of non-ferrous metals.This divisional application deals with the modifications whereby themetals as sulfides are separately rather than jointly precipitated.

Several general considerations, applicable to the present invention,should be noted at this point. Where the relative solubility of asulfide is discussed herein, it is taken as indicated by the solubilityproduct K, i. e., the product obtained by dividing the product of themetal ion concentration and the sulfide ion concentration by theconcentration of the un-ionized sulfide in solution. For purposes ofcomparison, several values of the solubility product K for various metalsulfides, in water at 18 C., are listed in the following table:

Table loin lAXlO- From the foregoing table, it will be readily seen thatFeS is more soluble than any of the other sulfides listed, with theexception of that of MnS. This fact is utilized in the present inventionin the following way. While MnS is more soluble than FeS, it is notpresent in most sulfides ores of non-ferrous metals in appreciablequantities, and economically, the content present therein may usually bediscarded in the ferrous sulfate liquor without appreciable loss.Further, manganese sulfide per se is neither common nor cheap. It is,therefore, not a desirable sulfide for use in the replacement reactionof the present invention. Iron sulfides are both common and cheap.Therefore, naturally-occurring sulfides such as FeSs, FeS and mixturesthereof make excellent sulfides to replace other sulfides from solutionin the present process.

If sulfide minerals, containing available iron sulfides, are added tothe hot pregnant leach liquor containing sulfates of such metals ascopper, nickel, or zinc, whose sulfides are less soluble than FeS, irongoes into solution as ferrous sulfate. At the same time, replacement andprecipitation of an equivalent amount of one of the other metals occurs.This metal is always found to be preferentially that which has the nextmost insoluble sulfide.

By way of further illustration, it may be assumed that an ore containingcopper and iron sulfides is to be leached with acid-ferric sulfatesolution, with oxidation. A pregnant leach liquor, containing copper andiron sulfates, is obtained. If this solution is then treated withadditional amounts of the ore concentrate, similar reactions to thefollowing would be expected to take place:

Thus, when CuFeSz is added to a hot acid solution of copper sulfate, HzSis produced according to (1). The first of this His is used up accordingto (4), After a 4 of the Fea(SOs)s has been converted toFeSOs, theadditional His is used to precipitate copper according to (2) and (3).These or similar reactions will conh'nue until all of the copper isprecipitated from solution as CuS or CtlaS. Further, if other metalsalts, such as nickel, cobalt, zinc, lead, or cadmium and the like, theequivalent metal sulfides of which are more soluble than Oils, are alsopresent in the solution, substantially none of their sulfides will beprecipitated under equilibrium conditions, unless and until all thecopper is replaced and precipitated as its sulfide. This device is usedto separate and purifycopperfromalltheremainingmetals.Accordingly,aswasnoted,thesemetalsarelostfromthesystem in solution withthe ferrous sulfate. I

Although the present invention is not intended to be limited to aparticular theory of operation, it is believed that the incompleteprecipitation of sulfides other than copper is due to the fact thatoxidation reactions, such as that of Equation 4 above, destroys the (8*)ion before their concentration becomes great enough to permit thesolubility product for one of the other metal sulfides to exceed itssolubility constant. For example, inthecaseofnickelsulfide, thesulfideiomaredestroyed before the product of their concentration,multiplied by the concentration of the nickel ions, and the productdivided by the concentration of the NiS ions in solution, becomes greatenough to exceed the above-noted value, l.4)(l0-. Above this value ofthis constant, NiS will precipitate as a solid. In the process of thepresent invention, precipitation of all metal sulfides less soluble thanthe replacing sulfides is caused, rather than prevented, as waspreviously done.

Itisalsoaprincipalfeatmeofthepreaentinventionthatanaddedreducingagentisused,withtheaddedmetalsulfide,inthereplacementreactions. Thus,anacidsaltsolutionbeingtreatedwithanaddedmetalsnlfidesochasironsulfides,issimultaneouslysubiectedtotheaction of a reducing agent.It is believed, although once again not intending to limit the inventionto any particular theoryofoperatiomthatthistendstoreversetheoxidationreactions, such as that of Equation 4 above, and therebyincreasesthemlfidesionconcentntioninaolution. In this way, by suitablycontrolling additions, the replacement reactions may be controlled tocompletely precipitatesulfidesofany,orall,ofthoaemetalstheuilfatesofwhich are in solution, but the sulfides of which are less soluble thanthe replacing metal sulfide, i. e., FeS, i. e., have a lower K valuethan does the latter.

An ore concentrate ordinarily forms the feed material for the process ofthe present invention. It may have been obtained by froth flotation,gravity separation, or any other conventional method, whereby a bulk ofthe gangue constituents is For purposes of illustrative discussion, itmay be considered that the representative ore concentrate is onewhichcontainsiron,copper,niekel,cobalt,zine,and manganese only. Otherconstituents, such as silver, lead, molybdenum and the like, will nodoubt be present, at least in small amounts. However, the amountsthereof areeithernnallenoughtobediscardedeconomicallyortheirbdiaviorandtreatmentwillbeshownbyoneofthe illustrative metals. Thiswill depend on whether, like manganese, the sulfides are more solublethan FeS or, like copperandzinc,thesulfidesarelessaoluble. Theaefactsbeing true, it simplifies the further discussion of the presentinvention to consider the problem as if these additional metals were notpresent. After the discussion has been developed, it is believed thattreatment for any one oftheseparticularsulfides,ifdesirable,willhavebeenclearly indicated.

An ore concentrate of the above-noted illustrativenamreisfirstsenttosometypeofleachingsystem. The invention is notparticularly concerned with the arrangement of the leaching operationand apparatus. It is quite p sible to adapt the overall process to anyof the various acid, acid sulfate and ammoniacal leaching systems orprocedures which are now well known.

However, it will be brought'out that the replacement reaction of thepresent process is initiated, and usually is carried out, in an acidsolution. For other than acid leaching, therefore, adjustment of the pHof the leaching solution must be carried out before the replacementreaction is carried out. The latter must be done on a solution ofsoluble salts, preferably the sulfates. Therefore, while sulfates, orsome other equivalent salt, are readily formed in solution afterleaching, regardless of the exact leach liquor used, acid sulfateleaching is usually the most desirable and will be generally taken asbeing illustrative in the following discussion.

Whatever the leach liquor used, it is added to the ore concentrate inthe leaching system. This may be as acid leach liquor. Preferably, butnot necessarily, air or oxygen or oxygen-enriched air is blown throughthe mass being leached. An acid sulfate leach liquor is also probablypreferably, because it is adapted to the treatment of a greater varietyof non-ferrous metal ores, including both sulfides and oxidized metalminerals.

Specific details of the leaching operation are not an essential featureof the process of the present invention. Leaching is carried out in someknown manner, according to conventional practice. Usually, but notnecessarily, it is carried out at elevated temperatures. An acidleaching with concomitant oxidation is highly exothermic. Due to thepreferred use of air of oxygen, it is therefore preferably carried outunder increased pressure. Conventional apparatus is usually availablefor the purpose.

The discharge from the leaching system proper comprises a slurry. Thelatter will contain undissolved solids, principally gangue, and asolution of salts of the mineral values.

If such metals as lead, whose sulfates are insoluble in the leachingsolution, are also present in the ore concentrate, these will be removedwith this residue. This residue, normally, is sent to waste. Variouscirculating systems, to insure against metal losses in the leachingsteps, are conventional.

Although, as noted above, acid leaching is preferred,

' if a basic leaching system is to be used, the pregnant leach liquor,ordinarily, will be converted to the acid side before the replacementreaction. While a replacement reaction may be carried out in a basiccircuit, such a procedure will, ordinarily, involve a considerableamount of additional apparatus, require additional reagents and veryappreciably complicate filtration. Accordingly, replacement, as notedabove, is preferably carried out in acid solution. Acidification forthis or other reasons may be carried out, either before or afterfiltration.

Acidification, if necessary, may be accomplished by the direct additionof acid. Preferably, however, it should be done by using the basicpregnant leach liquor as part of the leach liquor in carrying out asupplemental oxidation leach, with additional amounts of sulfide ore. Inthis way, neutralizing acid may be formed in situ.

As a result of the above procedures the metal values are now present inthe filtrate as a solution of soluble sulfates. This solution is sent tothe primary replacement reaction. Here, as noted, it is treated with themetal sulfide and a reducing agent. Replacement may be carried out inany suitable vessel, equipped to carry out chemical reactions underpressure. For example, a solution of Cu, Co, Ni, Zn and Mn sulfates maybe treated with added iron sulfide. This is preferably from someseparate source as fairly concentrated FeS. However, if "so desired anequivalent amount of the same ore concentrate fed to the leaching systemmay be used. In some cases, it may be desirable to use a mixture, partlyore concentrate and partly FeS from some such separate source, such aspyritic iron ore.

Whether used from a separate source of iron sulfides,

This slurry is filtered to remove the residue. The latter is principallygangue and an acid-insoluble gangue.

- temperatures.

or as part of the ore concentrate, or as a mixture, there must beavailable iron and available sulfur present. The total amount of addediron and sulfur should be at least slightly in excess of the theoreticalequivalents required to precipitate sulfides of all the replaceablemetals whose sulfates are in the pregnant leach liquor.

Ordinarily, the reaction will be carried out at elevated Usually thiswill be in the range from about 275-750 F. While higher temperatures maybe used, there is no particular advantage in so doing. While the lowertemperature range places the least restrictions on the apparatus at thelower temperatures, considerably longer periods are required. If anoxidizing acid leach is used, the reaction is exothermic. Also, there isno trouble in obtaining the elevated temperature which is preferable inthe replacement reaction. The sulfates solution, coming to thereplacement reaction, is already hot. If additional heat is required forfurther temperature rise, it is readily available from the waste heatthat is ordinarily removed by blowing steam from the oxidizing leachingsystem, to prevent the temperature of the latter from becoming too high.

For the same reason that elevated temperatures are preferred, apparatusequipped for agitation is also desirable. It is not essential tosuccessful operation. However, in general, a replacement which can becarried in one and one-half to two hours at about 450 F. without violentagitation, can be carried out in about one-half hour, at the sametemperature, if the pressure vessel is equipped for additional agitationof its contents.

Supplementing the action of the added solid sulfides is one of thecritical features. For this purpose, as noted above, an additionalreducing agent, preferably a gas, is used. Substantially any availablereducing gas may be made to serve the purpose. Carbon monoxide, sulfurdioxide and the like, may be used, for example. Hydrogen is, perhaps,even better, as it is an excellent supplement to the hydrogen sulfidewhich is always liberated during the replacement reactions. Mixtures ofcarbon monoxide and hydrogen are found in various industrial gases, andare usually the most economical and the most readily utilized.Hydrocarbons, such as methane and ethane, may be used. However, theiruse alone is not too desirable, because, in some cases, they appear toform complex ions with some of the metals. Their presence, or thepresence of sulfur-bearing gases, as part of an otherwise desirable andavailable gas mixture, does not appear to be harmful, in this respect.

The actual consumption of reducing agent is comparatively small. It isnecessary only to maintain suflicient concentration to retard or reversethe tendency toward completion of oxidation reactions, such as thatdiscussed above. For this reason, if desirable, or necessary, other andless economical agents may be used. For example, methyl and ethylalcohol may be used for the purpose. Formic acid, oxalic acid, and thelike, formaldehyde in its various commercial forms, and as itssulfoxylates, serve the purpose. Ordinarily, however, the use of a gaswill be found more desirable, physically and economically.

Treatment with the added reducing gas, or other agent, and the ion orother metal sulfides is continued until substantially all of theprecipitatable sulfides less soluble than the treating sulfide, i. e.,the FeS, have been precipitated. The resultant slurry is filtered. Thefiltrate is ordinarily removed from the system as an iron discard. Theiron, which will be principally present as ferrous sulfate, is theprincipal diluent metal.

In addition, this solution will contain sulfates of those metals whosesulfides are equally, or more, soluble than FeS. In the illustrativecase, these metals are represented by the manganese. When they arepresent in the ore, the filtrate, therefore, will also include suchvaried metals as magnesium, aluminum, chromium and the like. These areminor constituents and are present in very small spasms 7 amounts. Thecurrent practice in treating these ores is to discard these minorfractions. There is no reason why this practice should not be continuedin the operation of the present invention, unless an exceptional ore isfound. In the latter case, the content ofmanganese or molybdenum or thelike may be sufficiently high as to warrant special treatment.

The presscake from this filtration will contain any slight excess ofiron sulfides over that required to precipitate the less solublesulfides. In addition, it will contain sulfides of all the metals thesulfides of which are less soluble than FeS. In the illustrative case,these will be the sulfides of copper, nickel, cobalt and zinc. Whilethis mixture of sulfides will contain some iron sulfide, as did theoriginal ore concentrate, the proportions are entirely different. Thesmall amount of iron remaining is readily removed. In some cases, theleaching operation may not produce a solution in which the ratio ofother metal sulfates to iron sulfate is sufficiently high for areplacement operation of optimum efficiency. If so, a part of thepresscake from the filtration may be diverted and returned to theleaching system to supplement the valuable metals content of the oreconcentrate being fed thereto. In this way, the solution coming from theleaching operation may be given any desirable ratio of desirable metalsto iron. It is found that from about :1 to about 20:1, as ratios insolution, is a good general practice.

As will be seen from the foregoing discussion, replacement by means ofthe added metal sulfide and the reducing agent can be used to separatethe metal values sulfides substantially completely from the diluentmetals and gangue in the original ore. In some cases, it may bedesirable to insure complete precipitation without having present theseslight excesses of the metal having a sulfide of sufficient solubilityto be used in the replacement. If such complete precipitation is carriedout, the concentrate will necessarily contain the excess unreactedreplacement sulfide. However, instead of having a metals values todiluent metals ratio of 1:1 or lower, as frequently found in an originalore concentrate mixture, the product sulfides mixture is nowsubstantially free from iron.

When it is desirable to have this mixture completely free fromreplacement sulfide, this is accomplished by using two replacementreactions. In the first, slightly less than the amount of added moresoluble metal sulfides than is stoichiometrically required to replaceall the other less soluble sulfides is used. Precipitation will thencease when the iron or other replacement sulfide is used up, rather thanwhen the less soluble sulfides of the replaced metals are completelyprecipitated. The soreplaced sulfides, free from replacing sulfide,which, in the illustrative case means free from iron, may then becollected in any desired manner, as by filtration.

The remaining solution is then subjected to complete precipitation withan excess of added metal sulfide, i. e., iron sulfide. This second, orsupplemental, replacement produces a sulfides concentrate which is smallin amounts and contains added metal sulfides as well as replaced metalsulfides. This concentrate is collected and is either recycled to theleaching operation, or added with the slurry coming into the primaryreplacement stage. The latter operation is, perhaps, simpler.

It might appear that, after stripping the sulfides with a deficiency ofsoluble metal sulfide, and removal of the desired sulfides concentrate,the residue could be returned directly to the replacement reaction whichis designated as a primary replacement. This cannot be done, because,except in special circumstances, such a practice would build up anexcess of dissolved iron sulfate, or its equivalent, in the replacementcircuit.

It is, of course, much simpler, and will presumably be preferable, toprecipitate all the mixed sulfides by using a slight excess of ironsulfide, or its equivalent. The resultant small amount of iron sulfidecontaminant is readily removed later. This was noted above and will beexplained more fully below.

A second principal operation of the present invention operates in theseparation of the sulfides mixture concentrate into its componentsulfides. Commercially, of course, any process which cannot accomplishthis result is not particularly useful. In the past, little helpfulinformation has beenavailable as to processes suitable for the purpose.In general, they were largely confined to two fields, fractionalcrystallization and selective leaching.

A substantially complete solution of all the recoverable metal values inan ore is almost a requisite for economical operation. Unfortunately,such a mixture of constituents, as is found in most sulfide flotationconcentrates, is completely dissolved only by means of acid oracid-ferric sulfate leaching, generally with oxidation as an addedrequirement. This is the operation preferred in the present invention.Pregnant leach liquor soobtained can, and often will, contain, forexample, iron, copper, cobalt, nickel, manganese, zinc, molybdenum,silver, arsenic, tin, bismuth, calcium, magnesium, selenium, so; diumchloride and other minor items.

Discounting the iron, where in copper recovery is usually a diluent,metal values of copper, nickel, cobalt,

and possibly silver, zinc, and lead, are the only constituents likely tobe present in amounts which warrant an attempt at their recovery by thepresent process. As noted above, the remainder is usually present insmall amounts only and, in general, may be discarded. If, occasionally,one or more of these constituents is present in industrially-recoverableamounts, a special circuit can be set up for it. Otherwise, in thepractice of the present invention, the custom used in the presentindustry of discarding these minor constituents is generally followed.Reverting to the pregnant leach liquors, the illustrative valuable metalconstituents are in solution as soluble salts, generally as sulfates.unquestionably, the solubilities of these salts is such that little, ifany, benefit can be obtained by attempting to separate them byfractional crystallization. Particularly is this true in the ratios inwhich they are usually present in the leach liquors. Industrially, verylittle has been accomplished in this field, principally because of theobvious limitations.

The other alternative, selective leaching, can be successfully carriedout under certain conditions. Using very carefully controlled leachingon highly suitable ores, those ores containing only metals whosesulfides differ widely in solubility and in amount, selective leachingcan be employed. Again, unfortunately, such suitable ores are notcommon. Further, the careful leaching conditions are difficult toestablish and to maintain. Even more unsatisfactory, complete leachingof all the valuble metal substituents is seldom, if ever, possible.

It is, therefore, no small feature of the present inven' tion that itmay be readily employed to separate such a sulfides mixture as isproduced by the processes described above into its components. This isdone by properly controlled replacement.

The mixed slurry containing unreacted iron sulfide is subjected to anacid oxidation. For this purpose, the slurry is combined with a suitableacid, preferably sulfuric acid, since this sulfate is a desirable salt,and subjected to oxidation by blowing therethrough air, oxygen, oroxygen-enriched air. The reaction is carried out, preferably, in themanner used in the conventional oxidizing acid leaching, discussedabove. It is carried out under pressure at an elevated temperature ofabout 250- 750' F. Since the reaction is exothermic, there is nodifiiculty in obtaining either the pressure or the temperature. Usually,it may be necessary to bleed steam from the operation, in order toprevent excessive temperature and pressure loads upon the apparatus. Thelowest practical temperatures and pressures are preferable, as theydecrease corrosion problems in the apparatus.

The resultant solution of soluble sulfates is also filtered to removeany insoluble residue. Ordinarily, thiswill only result in the removalof discardable waste material. In some cases, however, the presscake maycontain a small amount of the material which it is desirableto recover.Usually, this is due to incomplete oxidation in the preceding step. Insuch cases, this residue is readily passed back to the oxidation step,or to the leaching system.

The resultant filtrate is subjected to the first of the actual sulfideseparating operations. It is accomplished, preferably at about 250-450F., by treating the filtrate in a suitable vessel with some metalsulfide which is more soluble than copper sulfide. Again, forillustrative purposes, this is considered to be iron sulfide. In theactual operation, it will be iron sulfide per se, in most cases, becauseof the large amounts readily available from pyritic iron ores. It may beore concentrate, if necessary or desirable. An amount stoichiometricallyequivalent to the available copper in the filtrate is added thereto.Replacement continues until precipitation of the copper sulfide issubstantially complete.

Solubilities of various metal sulfides is markedly affected by theacidity of the solution in which it is attempted to dissolve them. Inthe present case, it is desirable to completely precipitate coppersulfide, while maintaining in solution all the other metal sulfides. Theresultant iron sulfate should also be retained. At a pH above about2.7-3.0, other metal sulfides, such as cobalt and/or nickel sulfides,tend to precipitate with the copper, and a pH of about 2.7 is,therefore, about the maximum desirable limit and 3.0 is about the limitpermissible for suitable operation. On the other extreme, too high aconcentration of acid makes it quite difiicult to precipitate coppersulfide. About 15% acid content in solution is approximately the highestacidity which is desirable in this circuit.

Since it is undesirable to precipitate additional sulfides in thisparticular step; in fact, to the contrary, it may be desirable, and itis quite feasible, to add small amounts of air, oxygen, or ferricsulfate to prevent other metal sulfides from being precipitated with thecopper; it is apparent, then, that in this one step, the added reducingagent is'not necessary. However, added metal sulfide is used in usuallysubstantially sufiicient amount to insure, in and of itself, replacementand precipitation of all the copper as copper sulfide. This is collectedas product.

The filtrate, remaining from the removal of the copper sulfide, ispassed to the second of the sulfides separation operations. As shown inthe table of the solubility products, above, the next least solublesulfide, in the illustrative case, is that of cobalt. Accordingly, it isthe next to be removed. Suitable acid conditions for its completeprecipitation are in the pH range of about 3-5. Under favorableconditions, this range may be increased to 2-5 or 25.5. Reduction in theacid content of the solution to the requisite pH may be carried out inany desired manner. Probably the most simple method is the addition ofaqueous ammonia, or an equivalent base, which will not causeprecipitation of insoluble metal salts. However, it is usually much moreeconomical to utilize lime, or some similar alkaline-earth metal oxideor hydroxide. In the latter cases, however, the resultant insolublealkalineearth metal sulfate must be removed to prevent contamination ofsubsequently-precipitated metal sulfides. At the end of the copperprecipitating period, the copper sulfide is filtered out, lime is addedto the filtrate, and the resultant calcium sulfate precipitate isfiltered out. The calcium sulfate may be discarded.

Iron sulfides, as the illustrative added sulfide, are then added inequivalents to the cobalt sulfate, the equivalent sulfide of which is tobe precipitated. Replacement is carried out until precipitationsubstantially ceases. Again, the slurry is filtered and the cobaltsulfide is collected.

In a similar manner, subsequent replacement operations are carried outto successfully precipitate sulfides of the remaining dissolved metalsalts, i. e., those of nickel and zinc;in the illustrative case. Themost favorable pH conditions for replacement of the nickel sulfide isbelieved to be about 5.0-6.2. Zinc sulfide is most readily pre cipitatedat a pH of from about 6.2 to approximate neutrality. Care should betaken not to pass the circuit appreciably to the basic side at thisstage. If such precaution is not taken, it will be found that ironsulfide is not soluble. The replacement, using iron sulfide, therefore,is inoperative.

Because definite pH limits to be ,used in the present process have beenspecified, does not necessarily mean that a sulfide replacement for anyone metal cannot be carried out at other pH ranges, particularly whenother metals are not present. The ranges indicated here are those whichhave been found suitable for the separation operations. Also, it shouldbe noted that the reaction is being carried out under reducingconditions. Therefore, where the metal sulfide or sulfides beingprecipitated are of a metal or metals commonly exhibiting differingvalences, all or a part of the precipitated sulfides may be of the metalor metals in their reduced form.

The final filtrate, which may contain some small amounts of metalvalues, usually is of insufficient economic value to warrant furthertreatment. It is, ordinarily, sent to discard.

Again, it should be noted that, as discussed above, the illustrativecase is limited to certain metals. Others may possibly be present insufficient amount to be economically recoverable. In that case, aseparate circuit, therefore, should be set up, utilizing the principlesdisclosed with respect to the illustrative metals.

Purity of the precipitated sulfide depended upon using stoichiometricquantities of iron sulfide. As a practical matter, this is somewhatdifiicult to control. A slight deficiency in the amount of iron sulfideto be used to precipitate a different, particular sulfide, suchas thatof copper, has two effects. Copper will be precipitated as pure coppersulfide; unfortunately, however, not all the copper is precipitated, andthis residue is passed into the next stage and becomes a contaminant ofthe sulfide to be precipitated therein. Similarly, in each succeedingstep, there is danger of carrying over unprecipitated, potentiallyprecipitatable, metal, which will become a contaminant in attempting torecover the next sulfide to be separated.

On the other hand, the use of a slight excess of added sulfide over thetheoretical equivalents required has an entirely different, but equallyundesirable, efiect. The excess metal sulfide, not being reacted,remains as a solid in the slurry, and reports as a contaminant in thesame precipitation step. Either result is to be avoided, if possible. 7

By slight modification of the circuit, this ditliculty is readilyovercome. A mixed sulfates solution serves as starting material. Thissolution is then subjected to the first of a plurality of copper sulfideprecipitations. In the first, a deficiency of iron is used. As a result,incomplete. precipitation of copper sulfide is obtained. However, theprecipitated sulfide so obtained comprises substantially all the copper,and it is substantially pure, i. e., substantially free from other metalsulfides. This precipitate is filtered out and recovered as coppersulfide product.

The filtrate, containing the remaining metal values, and the smallamount of unprecipitated copper, is then given a second treatment toprecipitate the copper. In this second treatment, a slight excess ofiron sulfide is used. This precipitates all the residual copper sulfideas copper sulfide which is contaminated with the unreacted excess ironsulfide. It does not, however, constitute loss, because, as shown, thesolids can be recovered by filtration. The collected mixed sulfides,small ll in amount, are recirculated as part of the solids fed to thefirst copper sulfide precipitation.

As a result, the second filtrate is then reduced to a pH suitable forthe precipitation of the next most insoluble sulfide. Aqueous ammonia ispreferably used for this purpose, although, as discussed above, as apractical matter, it will usually be cheaper to use lime, burneddolemite, or the like, and a second filter. The pH-increased solution istreated to obtain a first cobalt sulfide precipitate, using a slightdeficiency of replacement sulfide over the theoretical requirements. Atthe same time, as in the foregoing discussion, a reducing gas, orequivalent agent, is used. The resultant precipitate of pure cobaltsulfide is collected by filtration.

The filtrate, containing a small amount of cobalt, is given a secondcobalt replacement; and, as in the case of 'copper, the remainingsulfide is precipitated and collected in a precipitate slightlycontaminated with ferrous sulfide. As was the case with the secondcopper sulfide precipitate, this second cobalt precipitate is returnedas part of the solids fed to the first cobalt sulfide replacement. Inthis way, as was the case of the copper, all the desired metal sulfideseventually report as a sulfide free from both diluent metals sulfides,sulfides of the metal values having a lower or a higher solubility thanthe desired sulfide, and from soluble salts.

The succeeding steps ran be carried out in the same way, modifying thenickel sulfide replacement into two stages, and the zinc sulfidereplacement into two stages. The final filtrate, after the zinc sulfidereplacement, is again discarded.

Sulfides other than iron, together with reducing gas, may be used toreplace those metals whose sulfides are less soluble than that of thereplacement metal. lllustra tive examples may be found in the followingrepresentative equations:

() NiS-l-CuSO4=NiSO4+CuS (6) ZnS-l-CuSOt=ZnSO4+CuS (7)ZnS+CdSO4=ZnSO4+CdS Reactions such as those illustrated above may beemployed to perform the replacement and separation of separate sulfideproducts from sulfate solutions, in accordance with the presentinvention. This may be done in one of several W s. The replacement, orreplacements, may be carr out on solutions obtained in other 12deficient to the equivalents of available, replaceable copper in thefiltrate. Replacement is carried out with these mixed sulfides. Theresultant slurry is filtered and the presscake from the filtration stepis the substantially pure copper sulfide product.

Filtrate is then treated with enough more of the dividedup sulfides toprovide a cobalt plus nickel plus zinc total slightly in excess of theremaining copper. In this reaction, the second copper sulfidereplacement, the remaining available copper is precipitated. There willbe precipitated therewith, the slight excess of mixed sulfides which areunreacted. Slurry is filtered, and the mixed sulfides are returned aspart of the feed to the first copper replacement. In this way,eventually all the copper reports in the pure copper sulfide fraction,and all the other metals continue into the subsequent treatments.

The filtrate, after the removal of copper sulfide and mixed sulfides, isdivided into two portions. Each portion is treated in a manner analogousto that used in the copper precipitation. One portion is used to obtainthe replacement sulfides to be used in the next step. The remainder isused as the solution, from which the next most insoluble sulfide isreplaced. The next step is the removal of cobalt.

The first portion of the divided filtrate is sent to a separatereplacement operation. Here, it is treated with processes, or they maybe carried out on solutions made expressly for the purpose. Preparationof the latter has been fully illustrated above.

It should also be noted that, in the operations discussed above asillustrative, stoichiometric quantities of iron sulfide were reactedwith the sulfates solution. Only the metal with the most insoluble metalsulfide is precipitated and filtered off. The acidic conditions weremaintained at the most favorable range for the purpose. The solutionswere then treated successively with additional quantifies of ironsulfide and additional quantities of reducing agent, therebysuccessively separate metal sulfides were replaced. This is repeateduntil the desired separations are accomplished.

In a modified procedure, the metal sulfides mixture, instead of beingdissolved, is divided into two portions. Only one part is oxidized toconvert the metal sulfides into dissolved soluble salts. The remainingportion is filtered and the presscake or residue is discarded.

Rather than using iron sulfide to replace the valuable metalconstituents, as mixed sulfides, the untreated portion of the sulfidesmixture is used to treat the filtrate as the replacing metal sulfide.The filtrate; containing copper, cobalt, nickel, and zinc, is sent tothe first copper replacement. Therein, a mixed metal sulfides portion,taken out as noted above, is taken of sufiicient weight to have a cobaltplus nickel plus zinc content slightly additional quantities of ironsulfide and a reducing gas, and enough is used to convert the totalcobalt, nickel, and zinc in solution to insoluble sulfides. Ironsulfide, for example, is taken as illustrative, because it is usuallythe most readily-available cheap sulfide for the purpose. Any otheravailable sulfide may be used. It must, however, be able to replace lesssoluble, but desirable, metal sulfides.

In general, it is best to use a very slight deficiency of the ironsulfide. This is done to insure freedom from excess iron sulfide in thereplaced precipitate. If so de' sired, an arrangement may be used toprevent loss. Otherwise, the minor amount of unprecipitated cobalt,nickel, and zinc, may be discarded with the iron sulfate. The resultantprecipitate of cobalt, nickel, and'zinc is collected by filtration.Being free of both iron and copper, it is used as a replacement sulfidein treating the other portion of the filtrate to precipitate the cobalt.

The amount of sulfides mixture used for this purpose should have anickel plus zinc content at least equivalent to the cobalt in solution.The solution is reduced in pH acidity to the approximate value, as wasdiscussed above. As was also noted above, this may be done mosteconomically by using lime, or other alkaline-earth oxide or hydroxideand an extra filter. It is desirable to use the reducing agent at thisstage. The resultant slurry so obtained is filtered. Due to the use of aslight deficiency of replacement sulfide, a substantially pure cobaltsulfide product is obtained. Again, the filtrate is treated with aslight excess of the mixed nickel and zinc sulfides. The resultantprecipitation of cobalt sulfides, plus excess sulfides, as was thesecond copper precipitate, is recycled to the first cobalt sulfidereplacement.

The filtrate, which will then be cobalt and copper free, is passed tosucceeding treatments. In each succeeding stage, the same generalprocedure is followed. The filtrate is divided into two portions. One istreated with an extraneous sulfide, such as that of iron, to obtainprecipitating sulfides; and the remaining solution is treated to obtainthe product sulfide. It is believed that these successive treatments areapparent from the foregoing discussion. There should be sufiicient n'ncavailable in this way to precipitate the nickel. Since the solution,after removal of the nickel, will contain only zinc, and zinc cannot bereadily precipitated with zinc, some additional sulfide must be used forthe removal and collection of the latter. Again we prefer the ironsulfide. However, it is not necessary that it be used. Especially atthis stage, the solution is approaching neutrality. There is no reasonwhy other sulfides, such as sodium sulfide, could not be used to replacethe zinc sulfide.

As was noted above, replacement can be carried out in basic solution, ifso desired. In such a case is sodium sulfide, potassium sulfide, or thelike, which can be used as the replacement metal. By such a procedure,for example, it is possible to precipitate manganese sulfide from theferrous sulfate liquor obtained in the earlier stages of the process. Itis believed that, from the discussion of the principles of this case,there will be no difficulty in setting up a circuit for this purpose, ifso desired. Such a circuit, for example, is highly convenient if it isnecessary to recover a small amount of gold from the system. The goldsulfide is most readily precipitated from basic solution.

We claim:

1. In a process of separating therefrom as its sulfide at least onenon-ferrous metal content of an aqueous solution containing a pluralityof non-ferrous metal sulfates, the composition of which solutionprecludes effective separation by fractional crystallization, theimprovement which comprises the'steps of: treating said solution underacidic conditions in the presence of free acid with about astoichiometric equivalent at least one solid metal sulfide thesolubility of which is greater than the least soluble sulfide of anynon-ferrous metal the sulfate of which is dissolved in said solution andwith a reducing agent in sufficient amount to insure substantially nooxidation of sulfide ions to elemental sulfur; continuing the treatmentat from about 250450 F., until precipitation of said metal having saidleast soluble metal sulfide substantially ceases; and removing resultantreplaced sulfide precipitated by said treatment.

2. The process according to claim 1 in which the sulfide to beprecipitated is that of copper and treatment is carried out at a freeacid content range from that at conditions in the presence of free acidwith about a stoichiometric equivalent at least one solid metal sulfidethe solubility of which is greater than the least soluble sulfide of anynon-ferrous metal the sulfate of which is dissolved in said solution;continuing the treatment at from about 250 450 F., until precipitationof said metal having said least soluble metal sulfide substantiallyceases; followed by treating residual filtrate in at least onesuccessive but separate stage, in each stage treatment being with anadded increment of solid metal sulfides the solubility of which isgreater than that of the least soluble sulfide of any of the non-ferrousmetals the sulfate of which remains in solution and with a reducingagent in sufiicient amount to insure substantially no oxidation ofsulfide ions to elemental sulfur, in each stage, the freeacid content ofthe solution, being adjusted to and maintained within the range at whichthe metal sulfide would be precipitated, is substantially insoluble andprecipitation of sulfides of the residual dissolved metals is minimized,whereby the non-ferrous metals in solution as sulfates are successivelyprecipitated as their sulfides in substantially pure form, theprecipitated sulfide being removed between each stage.

4. The process according to claim 3 in which the sulfide to beprecipitated is that of cobalt and treatment is carried out at afree-acid content of from that at about pH 2 to that at about pH 5.5.

5. The process according to claim 3 in which the sulfide to beprecipitated is that of nickel and treatment is carried out at afree-acid content of from that at about pH 5 to that at about pH 6.2.

6. The process according to claim 3 in which the sulfide to beprecipitated is that of zinc and treatment is carried out at a free-acidcontent of from that at about pH 6.2 to that at about pH 7.

References Cited in the file of this patent UNITED STATES PATENTS1,193,734 Sulman et al Aug. 8, 1916 1,333,688 Sulman et al Mar. 16, 19201,869,259 Hughes et al. July 26, 1932 2,424,866 Udy July 29, 1947

1. IN A PROCESS OF SEPARATING THEREFROM AS ITS SULFIDE AT LEAST ONENON-FERROUS METAL CONTENT OF AN AQUEOUS SOLUTION CONTAINING A PLURALITYOF NON-FERROUS METAL SULFATES, THE COMPOSITION OF WHICH SOLUTIONPRECLUDES EFFECTIVE SEPARATION BY FRACTIONAL CRYSTALLIZATION, THEIMPROVEMENT WHICH COMPRISES THE STEPS OF: TREATING SAID SOLUTION UNDERACIDIC CONDITIONS IN THE PRESENCE OF FREE ACID WITH ABOUT ASTOICHIOMETRIC EQUIVALENT AT LEAST ONE SOLID METAL SULFIDE THESOLUBILITY OF WHICH IS GREATER THAN THE LEAST SOLUBLE SULFIDE OF ANYNON-FERROUS METAL THE SULFATE OF WHICH IS DISSOLVED IN SAID SOLUTION ANDWITH A REDUCING AGENT IN SUFFICIENT AMOUNT TO INSURE SUBSTANTIALLY NOOXIDATION OF SULFIDE IONS TO ELEMENTAL SULFUR; CONTINUING THE TREATMENTAT FROM ABOUT 250* -450* F., UNTIL PRECIPITATION OF SAID METAL HAVINGSAID LEAST SOLUBLE METAL SULFIDE SUBSTANTIALLY CEASES; AND REMOVINGRESULTANT REPLACED SULFIDE PRECIPITATED BY SAID TREATMENT.